From way higher up, massive block realized and rotated out from the face. I could see debris at the toe of the block ravelling off, sensing something big was about to happen. The sound was incredible. Anybody who has climbed Angel's Crest knows that the sound echos off across the North Gully and the North Walls. At first I saw the base of the block moving and dust clouds forming. I've watched more than my share of rock fall and landslide videos through school and work and for fun, but this was a first experience for me.

It was hard to judge how close it was to me. The view of the mountains was momentarily blocked by the free-falling granite. There was no rotation, just one monster chunk of granite aimed straight down. I looked up and Mark, jaw-dropped and speechless. Once the block hit the forest below, a huge plume of rock dust engulfed the area below. The volume of rock is estimated at 1000-5000 cubic metres. I couldn't see the parties below at this point but hoped that they were ok.

I think all that brown stuff is evidence that water and dirt was getting behind there for awhile. If you look at Dru's orange and green outlines, the rock fractures there are clean. So I think that thing was probably just sitting there being held by the flakes and pillar below it.

The upper right corner of the block fracture looks fairly clean too so, that was probably holding it as well.

Hard to blame this one on rainfall and pore pressure along the fault since we're in the middle of sunny and dry conditions.

Given the dirt behind the main scar, I think root wedging from the trees on the ledge might be to blame. We're in the middle of spring and the trees are in the middle of their annual growth spurt, putting on mass in their roots too.

Of course, the response time to glacial unloading can be tens of thousands of years, so in our time scale, things can fall off for no immediate cause. They are responding to an event (deglaciation) thousands of years ago.

Hydraulic conductivity varies along joints too. But it's hard to state with a straight face that the pore pressure varies enough that this thing popped off now in response to record rainfall months ago.

You guys are up against the problem we often deal with in Yosemite in terms of rockfall triggering, namely that several possible environmental triggers are present (water, root wedging, etc.) but there is no "smoking gun". Plenty of Yosemite rockfalls occur on bluebird days when you just wouldn't expect it.

The fresh scar in Mike's photos clearly shows weathering and staining indicative of probably decades of seepage, plus some residual soil. This suggests that the block was partially detached for quite awhile.

However, I'm most intrigued by the reports of precusor rockfalls that occurred in the cleaner rock beneath the big block; if that was indeed the case, the partial detachment and weathering of the big block may have been mostly irrelevant, as the block was undermined by smaller rockfalls that fell for some other reason.

In this case there definitely were two separate, smaller, audible rockfalls in the half hour before the big one.

It's also true that Paul M reported small rocks falling while new routing in the area over the last week.

This does point to progressive loss of support for the big block that ultimately fell, and it also suggests that frequent, small rockfalls can serve as a useful indicator for a future larger rockfall, which is an easy idea to convey to non-geoscientists.

I've never really heard rockfall while climbing on the Chief, so i would say it is definitely abnormal to hear it. If i heard stuff falling down like Richard did that morning i'd think twice about climbing there that day..